Watch together

Watch together puts one video on everyone’s screen in a call and keeps every copy on the same second. It lives on the call stage, rides the call’s signalling, and ends when the call ends.

What can be synced

Source	Sync	Notes
YouTube	play, pause, seek, speed	embedded from www.youtube-nocookie.com
Vimeo	play, pause, seek, speed	embedded from player.vimeo.com with dnt=1
Direct video file (.mp4, .webm, .m3u8 and similar)	play, pause, seek, speed	plays in a <video> element, best sync
Any other page	not possible	the start dialog says so and suggests screen sharing

Pages on other origins cannot be controlled from the client (same-origin policy), so they are refused up front rather than drifting. Screen sharing plays anything and contacts nobody outside the call.

Starting and consent

In a guild voice channel, starting a session needs STREAM. In a DM or group call, anyone in the call can start one. Anyone in the session can then play, pause, seek or change speed.

Before a player loads, the stage names the exact host it will connect to and what that host learns, and waits for you to agree. Consent is per session and held in memory only: it does not sync, and the next video asks again. No provider SDK is loaded at any point; the client speaks each player’s documented postMessage protocol directly.

Link cards in channels never become players. Outside a watch-together session, links still go through camo.

Sync behaviour

The room shares one timeline { seq, by, at, position, rate, paused }, where at is the publisher’s clock time when position was true. Each client measures its clock offset to the publisher and corrects drift:

Drift	Action
under 50 ms	nothing
50 ms to 1 s	adjust playbackRate (at most ±5%) to close the gap over 8 s
1 s or more	seek

Late joiners land at the current position from the first message. If two people press controls at once, the higher counter wins and ties go to the lower user id, so every client agrees without a coordinator.

If someone’s player stalls, the room pauses for them for up to 20 seconds, then continues and tells everyone who was left behind.

Privacy

Session notes travel over the call’s signalling route, one request per recipient:

POST /_telium/v1/channels/{channel_id}/voice/signal
POST /_telium/v1/conversations/{conversation_id}/voice/signal

Call	Who can read what you watch
DM or group call	nobody but participants: notes are sealed with AES-GCM under a key exported from the conversation’s MLS group
Guild voice channel	the guild server, same as the channel’s text. The start dialog and consent gate say so

Either way, the video service you agreed to sees your IP address for as long as the player is open. Every incoming note is validated, and a player source is always rebuilt from a parsed id, so a crafted note cannot inject a URL into a frame.

Operator control

The allowlist is published in the media registry:

GET /_telium/v1/media/registry  ->  { …, "watch_together": { "enabled": true, "providers": [] } }

[media_registry.watch_together]
enabled = true
providers = ["youtube", "vimeo", "file"]

• Omitting the section enables every service the client supports.
• providers can only narrow that list. An empty list means all.
• enabled = false turns the feature off, and the call offers screen sharing only.